Currently I'm building a lasertagger with Arduino as core. Most things are working adequately, so now I'm focusing on improvement. One of the things that got my curiosity is the following:

The tagger has a speaker to play sounds. This works fine. However: you can hear a faint clicking sound from the speaker whilst uploading a sketch to the atmega328. You get the same clicking sound while updating the display: I do this once every second, since it is supposed to display time in game. If I light up some leds with the mosfets on the board, you can hear the faint click as well. There must be some interference. The blue wire on the board connects the DAC to the TBA820M amplifier. The atmega reads data from the flash chip, sends it to the DAC, the DAC controls the input from the opamp, which sends it to the speaker. Everything works fine, I'm just wondering if I can eliminate the interference. Mind you: if I connect to blue wire to myself instead of the DAC, sometimes it functions perfectly as an antenna and I can hear a radio station really well.